Quick Facts

Heat waves and climate change

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Top Line

Human-caused global warming has increased the frequency, size, and duration of extreme heat events. What were once very rare events are now becoming more common.

Facts for Any Story

  • Extreme heat is the deadliest form of extreme weather in the United States, causing more deaths than hurricanes and floods combined; more than twice as many deaths as tornadoes; and more than four times as many as from extreme cold.1National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS), U.S. Natural Hazards Statistics, 2018. View Source
  • Heat waves are occurring three times more often than they did in the 1960s—about six per year compared to two per year.2U.S. Global Change Research Program Indicator Platform, Heat Waves. View Source Some recent evidence suggests the increase has been even greater.3Diffenbaugh, N. S., (2020) Verification of extreme event attribution: Using out-of-sample observations to assess changes in probabilities of unprecedented events. Science Advances, 6, eaay2368. View Source
  • Record-breaking hot months are occurring five times more often than would be expected without global warming, suggesting that 80 percent of such monthly heat records are due to human-caused climate change.4Coumou, D. et al. (2013), Climatic Change, 118(3-4). 771. View Source With continued climate change, the frequency of these intense events is likely to increase5Mann, M.E., et al., (2018), Projected changes in persistent extreme summer weather events: The role of quasi-resonant amplification. Science Advances 4, eaat3272. View Source with particular risks for some important food-producing regions.6Kornhuber. K. et al., (2019) Amplified Rossby waves enhance risk of concurrent heatwaves in major breadbasket regions, Nature Climate Change, 10, 48-53. View Source
  • Summer nights have warmed at nearly twice the rate of summer days in the United States7NOAA data show that summer nighttime lows have increased by 1.46 degrees F while summer daytime highs have increased 0.77 degrees F per century from 1895 when national records began to 2018. View Source —a shift that exacerbates health impacts because hot nights reduce the body’s ability to recover from hot days.8U.S. Centers for Disease Control and Prevention (CDC), National Environmental Public Health Tracking Network. View Source The percentage of the U.S. continental land area experiencing abnormally hot nighttime temperatures increased from 5 percent to 40 percent over the four decades between 1970 and 2010.9NOAA/National Centers for Environmental Information, Climate Extremes Index. (Specific Data.) View Source
  • In addition to more extreme heat, humidity is also rising in some regions like the eastern United States.10Seager, R. et al. (2015), Journal of Applied Meteorology and Climatology, 54, 1121. View Source11Raymond, C., et al., (2020), The emergence of heat and humidity too severe for human tolerance, Science Advances 6 (19), eaaw1838 View Source This takes an extra toll on health because humidity interferes with the body’s ability to cool itself through the evaporation of sweat.12NOAA, NWS, What is the Heat Index? View Source A temperature of 90 degrees F with 80 percent humidity feels like 113 degrees F.13Ibid. View Source
  • Those most vulnerable to heat-related illness and death include young children, the elderly, those with chronic illnesses, student athletes who practice outside, outdoor workers, city dwellers, and those without air conditioning (or who lose it during power outages).14CDC, Natural Disasters and Severe Weather. View Source
  • Heat waves are not just more frequent than in the past but on average affect a 25% larger land area in the Northern Hemisphere than they did in 1980; including ocean areas, heat waves grew 50% larger.15Skinner CB, et al. (in prep); see also AGU GeoSpace blog ‘Northern Hemisphere heat waves covering more area than before.’ 2018. View Source The Northern Hemisphere-wide string of extreme heat events in the summer of 2018 was the largest ever recorded and a new analysis concluded that it would not have occurred without human-caused global warming.16Vogel, M.M. et al. (2019), Earth’s Future, 7. View Source
  • Recent evidence suggests that climate change is altering atmospheric circulation, such as the jet stream, causing persistent weather patterns to get stuck in place, increasing the duration and damaging effects of heat waves.17Horton, D. E., Diffenbaugh. A. et al.(2015), Nature, 522, 465. View Source18Mann, Michael E. et al.(2017), Scientific Reports, 7, 45242. View Source

Pitfalls to Avoid

It’s no longer true that “no single extreme weather event can be attributed to human-caused climate change”—a common refrain in past coverage of heat waves and other weather extremes. With more than 200 peer-reviewed studies published to date, attribution science—which can indicate the contribution of human-caused climate change to individual heat waves and other extremes—is increasingly credible and deserving of careful coverage.


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Dr. Mona Sarfaty

Medical Society Consortium on Climate and Health


Creative Commons LicenseThe text and video on this page are licensed as Creative Commons CC BY-SA 4.0. Journalists are free to use any text or video on this page with or without attribution to SciLine.